Fuel injectors in internal combustion engines must be capable of injecting precisely controlled quantities of fuel into the combustion chambers of the engine. Each injector delivers fuel through an outlet valve, and as long as the outlet valve is fully open, the injector can be assumed to deliver fuel at a constant rate. If the valve were always either fully open or fully closed, then the quantity of fuel delivered would be strictly proportional to the period during which the valve is open. But in reality, the valve takes a certain length of time to open fully and consequently the proportionality remains strictly true only as long as the valve opens with the same rapidity each time.
In electromagnetic fuel injectors, the valve is opened by an electromagnetic solenoid coil. A coil of this kind exhibits a certain auto-inductance, with the result that the current flowing through the coil builds up following an exponential curve when a constant driving voltage is applied. The slope at the beginning of this curve is a function of the applied voltage. For rapid operation of the injector, the current in the solenoid coil should be allowed to rise quickly enough to produce a high magnetic flux in the magnetic core of the device at least sufficient to cause the armature of the device to start moving. The current is then allowed to rise to a peak value within a predetermined time period, during which the armature completes its movement.
Repeatability is also a requirement for electromagnetic fuel injector control systems. Being able to repeatedly transition from zero to a predetermined current level within a tolerance of several microseconds is a requirement for many fuel control systems. Such repeatability is typically achieved by using a boost voltage supply to drive the solenoid coil. The boost voltage supply typically consists of a DC-DC converter which stores energy in a capacitor at a fixed voltage. The boost capacitor is then discharged into the injector solenoid. Because the boost capacitor is always fully charged to a predetermined fixed voltage prior to discharge, the pull-in current waveform is very repeatable.
It has been found that a considerable performance benefit can be realized by double pulsing the fuel injection solenoid within a single cylinder cycle. This mode of operating an engine dictates that in some operating conditions it is necessary to energize two solenoids simultaneously or within a very short time period of one another. With the boost voltage supply and driver circuitry used in prior art systems, this is not always possible. For example, a typical prior art system will employ a boost capacitor that is charged to approximately 100 volts, and then discharged into a solenoid until the current has reached 7.5 amps. For a typical prior art fuel injector solenoid, the pull-in time to 7.5 amps is approximately 150 microseconds. It then takes several milliseconds for the boost power supply to refresh the boost capacitor to 100 volts. If an attempt to energize another injector is made during the boost capacitor "refresh" time, the pull-in time to 7.5 amps will be considerably greater than the desired time, and will vary depending upon the exact operating conditions of the system. Such inconsistency in fuel injector opening times is -unacceptable in most applications.
One possible solution to this problem is to use two identical boost voltage supplies, wherein one of these supplies should always be completely refreshed. The engine control module (E.C.M.) would then commutate the refreshed voltage supply to the fuel injector to be energized. In this manner, the second voltage supply could be refreshed while the other voltage supply is being utilized. However, this solution is undesirable due to the added cost and space required for the second boost voltage supply, and due to the added complexity required to commutate the two boost voltage supplies correctly.
There is therefore a need for a means to energize two solenoids simultaneously or within a very short time period of one another without requiring redundant voltage supplies. The present invention is directed toward meeting this need.